The present invention relates to a system and method for controlling the air-fuel ratio for an internal combustion engine emission control system with a three-way catalyst, and more particularly to a system for correcting the deviation of the air-fuel ratio during rapid acceleration of the engine.
Such a control system is, as in U.S. Pat. No. 4,132,199, a feedback control system, in which an oxygen sensor is provided to sense the oxygen concentration in exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the burned air-fuel mixture. The control system operates to actuate an air-fuel mixture supply means to control the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio according to the signal from the oxygen sensor.
The system may sufficiently control the air-fuel ratio during the usual operation of the engine. However, during rapid acceleration of the engine, the system cannot immediately respond to the variation of the air-fuel ratio of the mixture as described hereinafter.
If the engine is rapidly accelerated, the amount of induced air immediately increases by an increase of the vacuum in the intake passage. On the other hand, the air-fuel mixture supply means does not rapidly operate in response to the increase of the amount of induced air. As a result, the air-fuel ratio increases and consequently, a lean air-fuel mixture is supplied. The air-fuel ratio gradually decreases to a proper ratio as the speed of the engine increases.